Ion Mobility Spectrometry ("IMS") is generally well known as a qualitative analytical tool. Basically, IMS separates ions by differences in the time it takes them to drift through a gas at atmospheric pressure in an applied electrostatic field. A sample gas alone or in combination with a carrier gas is directed into an ionization region containing a source of ionization, typically a .beta.-emitter, and accelerated into a drift region where they are separated based on mass, charge and size of the ions. These ions are then registered by a detector such as an electrometer amplifier. An important variant of the IMS device is a transverse field compensation IMS which utilizes two electrodes to provide an analyzing region between the ionization zone and the ion detection region. The analyzing region is set to a selected set of potentials which permits certain ions to transverse the analytical region to reach a detector such as an electrometer.
Of particular interest is Russian Inventor's Certificate No. 966583 which uses a transverse field IMS to provide ion separation for qualitative analysis. Improved separation is achieved by impressing an alternating asymmetrical electric field in the analyzing region which is of fixed electrical polarity. As described therein, the ion velocity V.sub.d, the ion mobility k and the electric field intensity E are related by: V.sub.d =k * E. Ion mobility k can be expressed by the following equation: k=k.sub.o +k.sub.2 E.sup.2 +k.sub.r E.sup.4 +. . . (1); where k.sub.i 's are coefficients which depend upon the ion species under investigation. By impressing an asymmetric alternating electric field in the analyzing region, the mean drift velocity becomes: EQU V.sub.d =k.sub.o 1/T.intg..sup.t.sub.t+t E(t)dt+k.sub.2 1/T.intg..sup.t.sub.t+T E.sup.3 (t)dt+ (2);
where T is the period of the electric field, t is the current time and k.sub.o is ion mobility when E=0. In an asymmetrical alternating electric field it can be seen from expression (2) that the mean drift velocity depends on the quadratic and higher order terms of the expansion for k given by Equation (1). The inventors disclosed that the maximum drift velocity can be reached if E.sup.+ /E.sup.- =2, where E.sup.+, E.sup.- represent the amplitudes of the positive and negative polarities respectively.
In International Journal of Mass Spectrometry and Ion Processes, 128 (1993) pp 143-48, a method of ion separation in dense air-gas media is disclosed using high-frequency electric fields. The method disclosed is based upon the teachings of the Inventor's Certificate No. 966583, except for the use of a high-frequency (2 M Hz) asymmetrical electrical field and differently configured apparatus. The paper discloses a method for the detection of trace amounts of amines in gas air mixtures where the threshold detection of tertiary amines was from 3.multidot.10.sup.-11 to 3.multidot.10.sup.-10 g/liter. The time to record the spectrum under analysis was 10 seconds. The apparatus disclosed is stated to be capable of being made as a portable gas analyzer.
Other apparatus have been proposed. For example, U.S. Pat. No. 3,699,333 discloses a method and apparatus for sorting and detecting trace gases using ion-molecular reactions in a drift field located between an ion forming region and detection region. In U.S. Pat. No. 3,935,452 a quadrapole mobility spectrometer is described. This device utilizes a carrier gas mixed with gas and ions directed between the quadrapole electrode which has impressed therebetween a hyperbolic electric field.
More recently, improvements have been reported in the lower limits of detectibility for ion mobility instruments. In U.S. Pat. No. 5,218,203 a device is disclosed for restricting a sample gas from entering the drift region and limiting sample gas ions to such regions. This device preferably operates above atmospheric pressure.
Accordingly, it is an object of the present invention to provide a gas analyzer which can provide an extended dynamic range over a wide variety of ionic species. It is a further object of the invention to provide an IMS analyzer which provides a secondary and tertiary means of resolution. It is also an object of the invention to provide an ion spectrometer which can be made to be portable and detect trace levels of species in air at a threshold sensitivity of about 10.sup.-11 g/liter. It is also an object of the invention to provide an analysis of the species at a distance remote from the actual source of up to 10 cm or more. It is a further object of the invention to provide an analyzer for the remote detection of drugs or explosives or other chemicals for which detection is desired at very low threshold limits.